The present invention relates to a procedure and apparatus for measuring asphaltene precipitation.
Asphaltene is definable as the fraction of an oil which is insoluble in N--C.sub.5 and soluble in benzene. The definition is, in other words, operational. There is no stringent physical or chemical definition. The structure of asphaltene monomer varies from oil to oil, but is generally supposed to consist of aromatic macromolecules having a significant content of heteroatoms (N,S,O). The asphaltenes are presumed to exist in solution partially as monomers and partially as colloidal aggregates with a continuous size dispersion.
Experiments show that asphaltenes, either dissolved in an oil or in a precipitate, have a very wide range of sizes. Typical data shows a molecular weight distribution extending from about 1000 to over 200,000.
Precipitated asphaltene is a big problem in connection with exploitation and processing of petroleum products. Asphaltenes are deposited in valves, tubing and process equipment. At worst, these deposits lead to complete blockage and production stoppage, with substantial costs as consequences.
Whether asphaltenes precipitate from a petroleum product, e.g. crude oil, substantially depends upon the oil's pressure, temperature and composition. Problems with asphaltene precipitation are seldom expected. This is because knowledge about asphaltene precipitation is limited and models currently in use are only to some extent predictive.
Neither is there, as of now, on-line equipment for detecting or quantifying asphaltene precipitation. Conventional equipment is only applicable in a laboratory context, and is inapplicable for industrial process-control.
A conventional method uses a source of light in the form of a laser or the like with a light detector. Both are put in an oil sample diluted with a solvent. An injector adds a flocculant (a flocculating agent) to initiate flocculation. The flocculation point (the precipitation point) is determined as the light detector registers a reduction in transmitted light occurring when flocculation occurs. The method determines only the flocculation point (flocculation threshold value) and cannot measure asphaltene precipitate. Besides, the method is not selective and consequently does not indicate if the precipitate is asphaltene or another constituent, like sand, found in the oil.
Another conventional method for detecting the flocculation point for an oil sample is based upon the measurement of the surface tension at the interface between oil and water as a function of added quantities of flocculant. When precipitation occurs, the interfacial surface tension increases and the flocculation point can thereby be determined. However, the quantity of precipitated asphaltene cannot be determined by this method. Also, its accuracy and reproducibility are uncertain. Furthermore, the method is work-intensive and inapplicable for pressure testing.
These last two conditions apply also to a third conventional method, gravimetry, based on standard liquid chemistry. Oil and flocculant are mixed at a desired proportion to precipitate asphaltenes. The precipitate is centrifuged, washed and weighed.
This method is used only to determine the quantity of asphaltene precipitate in an oil sample and cannot give the oil's
flocculation point without a series of very detailed and time-consuming gravimetric analyses.
In summary, the conventional methods are only applicable in a laboratory context. Considering this, and the fact that currently there are no predictive models for determining asphaltene precipitation, the inventors decided to develop a method and an apparatus for measuring the precipitation and being used on-line for industrial process control and surveillance.